Lensometry gives the axis of the cylinder in the lens. Lensometry measures four properties of a lens: sphere and cylinder power in diopters, the cylinder axis, the presence and direction of any prism, and the optical centers.
Lensometry is the measurement of the power of a patient's existing lenses, and the ophthalmic assistant usually performs it. Reading the old glasses before refraction gives the doctor a starting point for the refraction. It also shows how much the correction has changed. It is used again on new glasses, to confirm they were made to match the doctor's prescription. The other choices are real measurements, but each needs a different tool. Corneal curvature comes from a keratometer. The curve on the front of the lens, the base curve, comes from a lens clock. Neither value appears on the power drum. Prism shows up in the lensmeter when the target sits off center at the point in front of the pupil. The optical center can be marked with the dotting device built into the instrument.
Match the tool to the measurement. The lensmeter reads what the lens does to light: power, axis, prism, and optical center. Anything about the shape or the thickness of the glass itself needs a different tool. Knowing the reach and the limits of the lensmeter is the starting point of the COA lensometry content area.
Incorrect Answers
- A. Corneal curvature is measured by keratometry, using a keratometer, and it says nothing about the glasses.
- B. Lens thickness is a physical dimension, not a property of the light leaving the lens. The lensmeter reports nothing about how thick the glass is.
- D. The base curve of the front surface is read with a Geneva lens clock placed against the lens.
Testing Pearls
- Lensometry reports sphere, cylinder, axis, prism, and optical center.
- The instrument is called a lensmeter, lensometer, or vertometer.
- Lensometry before refraction supplies a starting point and shows change over time.
- Corneal curvature needs a keratometer; the base curve needs a lens clock.
- The dotting device marks the optical center for checking against the pupils.
References
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 5, Optics and Refractive States of the Eye.
- American Academy of Ophthalmology. Ophthalmic Medical Assisting: An Independent Study Course. Chapter 12, Fundamentals of Practical Opticianry.
